2.4.3 Adding and Removing Labels

These functions add and remove value labels from a struct val_labs
object. These functions apply only to numeric and short string sets
of value labels. They have no effect on long string sets of value
labels, since these sets are always empty.

Adds label to in var_labs as a label for value,
which must have the same width as the set of value labels. Returns
true if successful, false if value already has a label or if
val_labs has long string width.

Adds label to in var_labs as a label for value,
which must have the same width as the set of value labels. If
value already has a label in var_labs, it is replaced.
Has no effect if var_labs has long string width.